1. Field of the Invention
The present invention relates to the field of light emitting diode drive circuits.
2. Prior Art
Light emitting diodes are used in many applications to light displays or buttons, such as on cell phones, pagers, computers, etc. In such applications, it is desirable to have a constant illumination when using one or multiple diodes. The brightness of a light emitting diode depends upon the current flowing through the diode. Diodes are conventionally biased through a series resistor from a regulated voltage supply. The problem associated with this technique is that the amount of current through the diode depends significantly upon the forward voltage drop of the diode, which varies with size, process, temperature and aging. The present invention is directed to methods and apparatus for providing a predetermined current to such diodes in spite of these variations.
Current source methods and apparatus for light emitting diodes providing constant diode current and illumination in the presence of voltage and process variations. The method comprises providing a predetermined current through a first transistor, and mirroring the current through the first transistor to at least one additional transistor while holding the voltage across the first transistor to a predetermined value, wherein each additional transistor is coupled in series with a light emitting diode. An exemplary circuit, as well as various illustrative applications are disclosed.